Sequence of Operation — Air Conditioning
Y call → contactor → compressor + outdoor fan → indoor blower. Short sequence, specific failures.
What you'll take away
- ▸ Walk through a residential split-system A/C call for cooling
- ▸ Understand how the 24V call energizes the contactor and starts the compressor circuit
- ▸ Identify the common failures at each stage
A residential split-system air conditioner’s sequence is shorter and simpler than a gas furnace. There’s no ignition, no flame, no inducer. What there is: a thermostat call, a contactor that switches high-voltage to the compressor, and motors that start running.
The sequence is straightforward. The common failures — compressor starting issues, low refrigerant, contactor problems — are where most calls go.
The sequence
Stage 1 — Cool call
Thermostat closes Y (and usually G for blower). 24 VAC appears at the Y and G terminals of the indoor control board.
Measure: 24 VAC at Y during call. Also G for fan. If missing: Thermostat or wiring fault.
Stage 2 — Indoor blower starts
G signal energizes the blower relay immediately (cooling doesn’t have a heat-off delay — fan starts with the call). Blower starts on cooling speed tap.
Measure: 120 VAC at blower motor during call; blower running. If missing: Blower relay, motor, or ECM communication issue.
Stage 3 — Y signal travels to outdoor unit
The 24V Y signal is transmitted via the low-voltage control wiring from indoor to outdoor, typically on a dedicated pair in the thermostat cable bundle or a separate low-voltage cable run. The outdoor unit’s contactor coil is connected between this Y signal and 24V common.
Measure: 24 VAC at the outdoor contactor coil during call. If missing: Break in the Y wire between indoor and outdoor, or thermostat common / 24V loss at the outdoor unit.
Stage 4 — Contactor pulls in
The contactor coil energizes. The contactor’s main line-voltage contacts close, sending 240 VAC to the compressor and to the outdoor fan motor.
Measure: 240 VAC across the contactor’s load-side terminals; contactor clearly pulled in (audible click). If missing: Contactor coil failed, contactor mechanically seized.
Stage 5 — Compressor and outdoor fan start
Compressor attempts to start. Outdoor fan motor starts (sometimes on the same contactor, sometimes with a separate capacitor circuit). On compressors with a start capacitor / start relay, the start winding is briefly energized to help the rotor accelerate.
Measure: Compressor running amps (via clamp meter) near RLA; outdoor fan running; condenser coil getting warm. If missing or stuttering: Compressor fault (mechanical or electrical), capacitor failed, start relay failed, low voltage to the outdoor unit.
Stage 6 — Refrigerant cycle establishes
Refrigerant circulates. Outdoor coil rejects heat to outside air. Indoor coil absorbs heat from the return air stream. Blower distributes cooled air through the house. Suction pressure and head pressure stabilize at their running values.
Measure: Suction pressure 60–75 psi for R-410A systems at design conditions; head pressure based on outdoor ambient; superheat 8–15°F; subcooling 8–15°F.
Stage 7 — Thermostat satisfied
Room temperature reaches setpoint. Thermostat opens Y (and G on most setups, though some leave fan running per setup). Contactor coil de-energizes. Contactor drops out. Compressor and outdoor fan stop.
Measure: 0 VAC at contactor coil; contactor dropped.
Stage 8 — Standby
Outdoor unit is off. Blower runs briefly or stops depending on thermostat fan setting. System returns to idle.
Common failures
Contactor won’t pull in. Either 24V isn’t reaching the coil (indoor-to-outdoor Y wire broken) or the coil has failed. Measurement: 24V at coil terminals during call. No voltage → wiring issue. Voltage present but no pull → coil dead.
Contactor pulls in but compressor doesn’t start. Check capacitor (most common), start winding, and compressor ground / continuity. A “hums but won’t start” compressor is usually a weak run capacitor or a failing start assist.
Compressor stutters / trips breaker. Low voltage under load, failed capacitor, shorted winding, or seized compressor. Measure running amps and voltage simultaneously.
Contactor contacts welded closed. Compressor runs even when thermostat is satisfied. Fix: replace contactor.
The LV cable between indoor and outdoor
One subtlety worth noting: the low-voltage cable running from indoor to outdoor carries the 24V Y signal, and usually a common return. Long runs, damaged cables, or field repairs that introduced high-resistance connections can cause low voltage at the outdoor unit’s contactor coil — just enough to partially pull the contactor in, or not quite enough.
Diagnostic signature: contactor chatters but doesn’t pull in firmly, or pulls in and drops intermittently. Fix: trace the LV cable, test connections, and if necessary run new cable.
Diagnostic shortcut: compressor amps tell refrigerant story
As noted in Chapter 16, clamping compressor running amps gives a quick refrigerant-side diagnostic:
- Below RLA: low charge or system starving
- At RLA: healthy
- Above RLA: overcharge, dirty condenser, weak condenser fan, or refrigerant-side restriction
This is a fast first-look before pulling gauges — and often tells you immediately whether refrigerant or airflow is the problem.
Check your understanding
0 / 301A residential AC call produces 24V at the indoor Y terminal but the outdoor contactor doesn't pull in. Your next measurement should be:
02An AC contactor 'buzzes' when commanded but doesn't fully pull in. What's the likely cause?
03You clamp compressor running amps on a call and read 21 A on a 14.5 A RLA compressor. The outdoor unit is noisy and hot. What's the most likely condition?
Before you close the chapter
You should now understand the AC SOO, know what to measure at each stage, and recognize common contactor and compressor failures. The next chapter covers heat pumps, which add reversing-valve complexity to the basic AC sequence.